Cellulose triacetate film is tough and has good dimensional stability, heat resistance, and optical isotropy. Therefore, it has been used in various applications such as a substrate for optical thin-film devices (e.g. a substrate for photosensitive materials), a polarlizer protective film (e.g. liquid crystal protective film) for liquid crystal devices, and color filters. Therefore, cellulose triacetate film is required to have satisfactory optical characteristics, for example, low yellowness index, haze, and birefringence and high transparency. When a solution (dope) containing cellulose acetate such as cellulose diacetate and a solvent is used in the production of fibers, it is required to have high spinnability.
To assure satisfactory optical characteristics, the cellulose acetate film is generally produced by the technology which comprises acetylating a linter pulp, a softwood pulp or a hardwood pulp, casting a solution (dope) containing the resulting cellulose acetate and a solvent on a support, and releasing the resulting film from the support.
However, when a linter pulp is used, the cost of the cellulose acetate is inevitably high. Moreover, when a high-purity softwood pulp among softwood pulps is used, there is a problem with steady supply of a raw material. When a low-purity softwood pulp is used, a transparency of a cellulose acetate tends to decrease. On the other hand, a hardwood pulp generally is advantageous cost-wise but is not satisfactory in the releasability of the cast film from the support so that it is difficult to attain a satisfactory surface smoothness and sufficiently high productivity. Furthermore, a cellulose acetate obtained from a low-purity pulp injures the stability of its dope or filterability and thus lowers the spinnability.